Port upstream heading insert menu labels

[Renakoni]

Upstream

Ports marktext/marktext#4854: marktext/marktext#4854

This is stacked on #69 (sync/upstream-4861-prism-dependency-order) only because the focused Muya test boots editor code that imports the Prism path, and Android main still has the pre-#4861 Prism dynamic import issue under Vitest.

Android relevance

Android carries Muya's paragraph quick insert menu and locale files. The menu is editor UI, so the terminology fix applies to the WebView editor as well: h1-h6 are CommonMark/HTML headings, not page headers.

Changes

• Rename the quick insert menu section from headers to headings.
• Rename display/i18n keys from Header 1 ... Header 6 to Heading 1 ... Heading 6.
• Update all Muya locale key names while preserving existing translated values.
• Extend quick insert menu search coverage to assert the new section name and heading titles.

Verification

• pnpm --dir third_party/muya exec vitest run src/ui/paragraphQuickInsertMenu/__tests__/search.spec.ts
• pnpm build